1. Field of the Invention
This invention relates to generally the field of light dimming, and specifically, to a ditch light flashing circuit for railroad locomotives.
2. Description of the Prior Art
Locomotives have numerous exterior lights serving various purposes. Among these are ditch lights which illuminate an area of the ground ahead of and on either side of the locomotive, that is, the "ditch" along the railroad right-of-way. Railroad safety regulations sometime require that the ditch lights flash to provide a warning to persons at grade crossings that the locomotive is approaching. Typically, the ditch lights are located near each corner of the locomotive to provide the warning in either direction of travel and on both sides of the locomotive.
Lights are typically flashed by alternately applying a voltage and interrupting the voltage. When the voltage is interrupted, the light assumes an "OFF" state. It has been found that the life of an incandescent filament lamp is shortened by the shock of repeatedly applying its rated voltage after extinguishing the lamp. For example, U.S. Pat. No. 4,904,998 to Niimi shows a digital flashing circuit using a timer and a counter. Lights are flashed by alternately fully energizing and de-energizing them.
To avoid this shock effect, resistors have been connected in series with the lamp to reduce its illumination to a dull glow during "OFF" periods. In AC systems, an inductive choke has been used. These dimming circuits waste energy dissipated from the resistors, or other devices. Headlights are also utilized at each end of a locomotive to illuminate the right-of-way ahead of the locomotive. The headlights generally do not flash, but may have several levels of illumination, for example, full, medium, and dim. To dim the headlights, resistors are commonly connected in series with the lights to reduce the voltage across the lights. The resistors dissipate energy when the lights are dimmed.
It is desirable to have a flashing circuit which does not completely interrupt the voltage to the lamp during an "OFF" state of the flashing sequence, thereby increasing the service life of the lamp. The use of dimming resistors should be eliminated to save energy and reduce heat. When applied to locomotive ditch lights, such a circuit should operate automatically at appropriate times during locomotive operation.
It would also be desirable to have a circuit which effectively dims a locomotive headlight without the need for voltage dropping resistors.
An additional problem is caused when the lamps fail. Incandescent lamps fail after many hours of use as the filament evaporates and thins down and a break occurs. This phenomenon is often experienced with household light bulbs which flash when a switch is turned on and the lamp fails. Electronic circuits, such as those which may be used to control flashing and dimming of locomotive lights, cannot withstand even a momentary overload such as might be produced when an incandescent lamp leads arc at the end of life. Most electronic circuits are much more sensitive to current overloads than a lamp fuse or a line current circuit breaker, causing the electronic circuit to be damaged when the lamp fails.